Method and apparatus for controlling the maximum output power of a power converter

ABSTRACT

An example control circuit for use in a power converter includes an input voltage sensor, a current sensor, and a drive signal generator. The input voltage sensor generates a first signal representative of an input voltage (Vin) of the power converter. The current sensor generates a second signal representative of a switch current through a power switch of the power converter. The drive signal generator generates a drive signal to control switching of the power switch in response to the first and second signals. The drive signal generator adjusts a duty cycle of the drive signal based on a product K×Vin×t to control a maximum output power of the power converter, where K is a fixed number and t is a time it takes the second signal to change between two values of the switch current when the power switch is in an on state.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/935,897, filed Jul. 5, 2013, now pending, which is a continuation ofU.S. application Ser. No. 13/235,284, filed Sep. 16, 2011, now U.S. Pat.No. 8,502,517, which is a continuation of U.S. application Ser. No.12/780,658, filed May 14, 2010, now U.S. Pat. No. 8,030,912, which is acontinuation of U.S. application Ser. No. 12/058,539, filed Mar. 28,2008, now U.S. Pat. No. 7,746,050, which claims priority to U.S.Provisional Application No. 60/922,191, filed Apr. 6, 2007, entitled“Method And Apparatus For Controlling The Maximum Output Power Of APower Converter.” U.S. application Ser. No. 13/935,897, U.S. Pat. Nos.7,746,050, 8,030,912 and 8,502,517, and U.S. Application Ser. No.60/922,191 are hereby incorporated by reference.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to power converters, and morespecifically, the invention relates to regulating the output of powerconverters.

2. Background

Many electrical devices such as cell phones, personal digital assistants(PDA's), laptops, etc. are powered by a source of relatively low-voltageDC power. Because power is generally delivered through a wall outlet ashigh-voltage AC power, a device, typically referred to as a powerconverter, is required to transform the high-voltage AC power tolow-voltage DC power. The low-voltage DC power may be provided by thepower converter directly to the device or it may be used to charge arechargeable battery that, in turn, provides energy to the device, butwhich requires charging once stored energy is drained. Typically, thebattery is charged with a battery charger that includes a powerconverter that meets constant current and constant voltage requirementsrequired by the battery. Other electrical devices, such as DVD players,computer monitors, TVs and the like, also require a power converter fordevice operation. The power converter in these devices also has toprovide output voltages and currents that meet the requirements of thedevice. In operation, a power converter may use a controller to regulateoutput power delivered to an electrical device, such as a battery, thatmay be generally referred to as a load. More specifically, thecontroller may be coupled to a sensor that provides feedback informationof the output of the power converter in order to regulate powerdelivered to the load. The controller regulates power to the load bycontrolling a power switch to turn on and off in response to thefeedback information from the sensor to transfer energy pulses to theoutput from a source of input power such as a power line.

The product of the power converter output voltage and current is termedthe power converter output power. In most power converter applications,it is necessary to limit the worst case maximum output power that can besupplied to ensure the device being powered is protected from excessivepower delivery. Improving the tolerance of the maximum output power thata power converter can deliver allows the electrical device being poweredby the power converter to be optimized for safe operation under faultconditions, improves the electrical device reliability and reduces theoverall cost of the electrical device.

One particular type of power converter that may be used is a flybackpower converter. In a flyback power converter, an energy transferelement galvanically isolates the input side of the power converter fromthe output side. Galvanic isolation prevents DC current from flowingbetween the input side and the output side of the power converter.

A flyback power converter produces an output by switching a power switchto store energy in the energy transfer element during an on time of thepower switch and deliver energy to a power converter output for at leasta fraction of the time the power switch is off. In a non-isolatedflyback converter, an energy transfer element is still required to storeenergy from the power converter input to be delivered to the powerconverter output, but no galvanic isolation is required to be providedby the energy transfer element.

In operation, a power converter may use a controller to regulate outputpower delivered to the load. More specifically the controller may limita maximum output power of the power converter in response to feedbackinformation derived by sensing output voltage and or output current atthe output of the power converter. Sensing output current at the outputof the power converter typically reduces power converter efficiencysince a resistive element is typically introduced to provide a voltagesignal proportional to the power converter output current. If the outputcurrent of the power converter is not sensed at the output of the powerconverter, the maximum power delivery limit is determined by thespecification tolerances of certain components within the powerconverter.

Two components whose parameters influence the tolerance of the maximumoutput power of the power converter are the tolerance of the inductanceof the energy transfer element and the tolerance of a protective currentlimit threshold for current flowing in the power switch while it is inan on state. The controller may sense the current flowing in the powerswitch while it is in an on state and may also set the maximumprotective current limit threshold. In this case the tolerance of thecontroller maximum protective current limit threshold will influence thetolerance of the maximum power converter output power.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 shows generally one example of a switching power converter havinga flyback topology and including a controller that controls the maximumoutput power of the switching converter in accordance with the teachingsof the present invention.

FIG. 2 shows representative switching cycles of waveforms from anexample switching power converter operating in accordance with theteachings of the present invention.

FIG. 3 shows example waveforms of inductance and current limitcorrection in accordance with the teachings of the present invention.

FIG. 4 shows two examples of curves displaying a relationship betweenpower switch switching cycle period and a product of a signalrepresentative of an input voltage to the power converter and the ontime period of the power switch in accordance with the teachings of thepresent invention.

FIG. 5 shows a flow diagram that describes an example method to regulatean output of a switching power converter in accordance with theteachings of the present invention.

FIG. 6 shows internal details of an example integrated circuit thatimplements a control technique in accordance with the teachings of thepresent invention.

FIG. 7 shows timing waveforms of the example integrated circuit of FIG.6 in accordance with the teachings of the present invention.

FIG. 8 shows a flow diagram that describes an alternate example of amethod to regulate an output of a switching power converter inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION

Examples related to controlling a maximum output power of a powerconverter in accordance with the present invention are disclosed. In thefollowing description, numerous specific details are set forth in orderto provide a thorough understanding of the present invention. It will beapparent, however, to one having ordinary skill in the art that thespecific detail need not be employed to practice the present invention.In other instances, well-known materials or methods have not beendescribed in detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment is included in at least one embodiment or example of thepresent invention. Thus, the appearances of the phrases “in oneembodiment,” “in an embodiment,” “in one example” or “in an example” invarious places throughout this specification are not necessarily allreferring to the same embodiment. The particular features, structures orcharacteristics may be combined for example into any suitablecombinations and/or sub-combinations in one or more embodiments orexamples. Furthermore, the particular features, structures orcharacteristics may be included in an integrated circuit, an electroniccircuit, a combinational logic circuit, or other suitable componentsthat provide the described functionality. In addition, it is appreciatedthat the figures provided herewith are for explanation purposes topersons ordinarily skilled in the art and that the drawings are notnecessarily drawn to scale.

As will be discussed, examples according to the teachings of the presentinvention include methods and apparatuses for controlling the maximumoutput power of a power converter without the need to sense outputvoltage and/or output current at the output of the power converter.Furthermore, examples according to the teachings of the presentinvention compensate for energy transfer element inductance toleranceand for the tolerance of a protective current limit threshold of thecurrent flowing in the power switch set by a controller. The eliminationof the need to sense the power converter output current at the output ofthe power converter improves power converter efficiency and reduces thepower converter component count leading to improved power converterreliability compared to known solutions. The compensation for energytransfer element inductance and controller protective current limitthreshold tolerances further improves power converter reliability andallows more compact and reliable design of the power converter and load.

In one example, the maximum output power of a power converter iscontrolled during each switching cycle of the power switch to ensurethat the power converter is responsive to changes in the energy transferelement inductance and the protective current limit threshold during theoperation of the power converter. By controlling the maximum outputpower of the power converter in this way, the power converter isresponsive to extreme operating conditions, such as very high ambienttemperatures that may not have been expected during the original designof the power converter, further enhancing the reliability of the powerconverter and load.

To illustrate, FIG. 1 shows one example of a regulated switching powerconverter 100, sometimes referred to as a power supply, in accordancewith the teachings of the present invention. In the particular exampleshown in FIG. 1, switching power converter 100 is a power converterhaving a flyback topology. It is appreciated, however, that there aremany other known topologies and configurations of switching powersupplies that could also control the maximum power converter outputpower in accordance with the teachings of the present invention, andthat the flyback topology shown in FIG. 1 is provided for explanationpurposes. It is noted that in other examples power converter 100 couldhave more than one output in accordance with the teachings of thepresent invention.

As shown, a control circuit 115 is coupled to a power switch 105, whichin one example is a metal oxide semiconductor field effect transistor(MOSFET), a bipolar transistor or the like. Power switch 105 is coupledto the input winding 103 of energy transfer element 109, which iscoupled to a DC input voltage V_(IN) 101 and an output power diode 117.In one example, DC input voltage V_(IN) 101 is the unregulated output ofa rectifier circuit coupled to a source of AC voltage not shown. Inputcapacitor 106 is coupled to power converter input terminals 190 and 191to provide a low impedance source for switching currents flowing throughfirst and second input terminals 190 and 191, energy transfer element109 winding 103 and power switch 105 when the power switch 105 is in anON state. In one example, control circuit 115 and switch 105 could formpart of an integrated circuit that could be manufactured as a hybrid ormonolithic integrated circuit. Control circuit 115 is coupled to receivea signal 114, which in one example is a voltage signal, but in otherexamples could also be a current signal, or other signal representativeof the power converter output and or input, while still benefiting fromthe teachings of the present invention.

In the example of FIG. 1, control circuit 115 is coupled to regulatepower delivered from the first and second input terminals 190 and 191 ofpower converter 100 to the power converter output terminals 192 and 193coupled to load 121. Energy transfer element 109 includes input winding103 and output winding 110 and an auxiliary winding 108. The signal 114is coupled to control circuit 115 from auxiliary winding 108 through theresistor divider formed by resistors 111 and 112. As shown in theexample, controller 115 includes a current sense circuit 140 coupled tosense a current through power switch 105, a sensor circuit 141 coupledto receive an input signal representative of an input voltage to powerconverter 100, and a timing and multiplier circuit 142 that processesthe output of current sense circuit 140 and sensor circuit 141.Controller 115 also includes an oscillator circuit, which is responsiveto timing and multiplier circuit 142, and a drive signal generatorcircuit 144 coupled to drive the power switch 105 into an on state foran on time period and an off state for an off time period. In oneexample controller 115 is coupled to adjust a duty cycle of the powerswitch 105 to be proportional to a value of the input voltage signalmultiplied by the time it takes for the current flowing in the powerswitch to change between two current values when the power switch is inthe on state.

The basic operation of circuit 100 will now be described with referenceto waveforms 200 and 201 in FIG. 2. In operation, control circuit 115regulates the output of power converter 100 by switching power switch105 in response to the signal 114. When switch 105 is on, energy fromthe input capacitor 106 is transferred into the inductance of inputwinding 103 of the energy transfer element 109. One example of a typicalcurrent waveform flowing in power switch 105 is shown in waveform 201 inFIG. 2. When power switch 105 is turned on at time 202, the current ID203 flowing through power switch 105 starts to increase. As shown in theillustrated example, the current ID 203 increases substantially linearlyafter power switch 105 is turned on. The rate of change of the currentwaveform 204 with time is given by:

dI _(D) /dt=V _(IN) /L _(I)  (1)

where V_(IN) is the input voltage 101 across input capacitor 106 in FIG.1 and L_(I) is the inductance L_(I) 198 of input winding 103 of energytransfer element 109 measured with all other windings of the energytransfer element 109 uncoupled from external circuitry. It is noted thatthe relationship in equation (1) does not account for any voltage dropacross power switch 105 or other second order voltage drops so as not toobscure the teachings of the present invention.

At the time 205 when the power switch 105 is turned off, the currentI_(D) 203 flowing in the power switch 105 has increased to a valueI_(Dpk) 206. The energy stored in inductance L_(I) 198 of winding 103 ofenergy transfer element 109 is given by:

E _(L) _(I) =½×L _(I) ×I _(DPK) ²  (2)

When the power switch 105 is turned off, the energy stored in theinductance L_(I) 198 of input winding 103 is transferred to the outputof the power converter 100 and a current that flows through a forwardbiased output power diode 117 to capacitor 118 and the load 121 coupledto the output terminals 192 and 193. While current flows through theoutput power diode 117 during the off period of switch 105, the outputvoltage V_(O) 119 across load 121 plus the forward voltage drop acrossoutput power diode 117 is substantially equal to the voltage across theoutput winding 110.

In some cases, the current may substantially stop flowing from outputwinding 110 through the output power diode 117 during the off period ofpower switch 105. In this case, the operation of the power converter isreferred to as discontinuous mode operation. In discontinuous modeoperation, substantially all the energy stored in the inductance L_(I)198 of input winding 103 of the energy transfer element 109 istransferred to the output of the power supply before the power switch105 is turned on again at the start of a next power switch switchingcycle. In the example of FIG. 2, the power converter is operating indiscontinuous mode since energy delivery period ted 207 is less thant_(off) 208. In power converters operating in discontinuous mode, thecurrent 203 flowing in the power switch 105 starts from a valuesubstantially equal to zero at the start of each switching cycle. If thepower switch 105 switching cycle period is T 209, the power delivered(P_(out)) to the output of the power supply is given by:

P _(out) =K1×½×L _(I) ×I _(DPK) ²×1/T  (3)

where K1 is a factor, less than 1, that accounts for energy lost in theenergy transfer from the input to the output of the power converter 100and can for example include losses in clamp circuit 102 that clampsenergy, often referred to as leakage energy, that does not couplebetween the inductance L_(I) 198 and the output of the power converter100. The term 1/T in equation 3 is often referred to as the power switchswitching cycle frequency, which in one example is determined bycontroller 115.

The maximum output power capability of a power converter operating inthe discontinuous mode of operation can be written as:

P _(outmax) =K2×½×L _(I) ×I _(DPKMAX) ²×1/T  (4)

where I_(DPKMAX) is a maximum protective current limit current thresholddetermined by controller 115. K2 may be a different factor than K1 ofequation (3) due to changes in the proportion of energy loss during themaximum load condition of equation (4) when compared to the loadcondition of equation (3). The tolerance of L_(I), I_(DPKMAX) and T fromone power converter to another will determine the tolerance ofP_(outmax) from one converter to another. In addition, the tolerance ofL_(I), I_(DPKMAX) and T in a single power converter under changingoperating conditions, such as the ambient temperature in which the powerconverter is operating, will also determine the tolerance of P_(outmax).

Now rearranging equation (1):

dI _(D) ×L _(I) =V _(IN) ×dt  (5)

Substituting values for the condition of a maximum power converter loadgives:

dI _(DPKMAX) ×L _(I) =V _(IN) ×dt _(onmax)  (6)

Therefore, any variation in either I_(DPKMAX) or L_(I) will result in achange in the V_(IN)×dt_(onmax) product according to the relationship ofequation (6). As will be described below, the controller 115 for examplecan be coupled to detect and measure both V_(IN) and dt_(ONMAX) in orderto generate an internal signal responsive to the V_(IN)×dt_(onmax)product and adjust a switching cycle period of the power switch 105 tobe proportional to the V_(IN)×dt_(onmax) product. In this way, the powerswitch switching cycle period is responsive to any change in eitherI_(DPKMAX) or L_(I) to reduce the variation in the maximum output powerof the power converter as a result of variation in I_(DPKMAX), L_(I) ora combination of both.

There are many ways that controller 115 may be coupled to receive asignal representative of the input voltage V_(IN) 101. In one example, adirect connection 130 is made between the input capacitor 106 andcontroller 115. In another example, controller 115 is coupled to detectcurrent I1 180 flowing out of terminal 123 during the period when thepower switch 105 is in the on state. During this period, the voltage atterminal 123 is clamped to be substantially the same as the voltage atterminal 107. This current I1 180 is therefore representative of theinput voltage V_(IN) 101 since the voltage appearing across auxiliarywinding 108 during the power switch 105 on time period is substantiallyequal to the input voltage V_(IN) 101 multiplied by the turns ratio ofN_(AUX) 171 to N_(I) 170. The choice of resistor 111 thereforedetermines the value of the current I1 180 flowing during the powerswitch 105 on time. Thus, current I1 180 is a signal representative ofthe input voltage V_(IN) 101. In one example current I1 180 can bewritten as

I1=KV _(IN)  (7)

where:

K={N _(AUX) /N _(I) }/R ₁₁₁  (8)

The relationship in equation (8) above assumes that terminal 123 issubstantially at the potential of ground terminal 124 when the powerswitch 105 is in the on state.

The waveforms of FIG. 3 are used below to describe examples of how therelationship of equation (6) may be used to control the maximum poweroutput of the power converter. In the example, the waveforms 300 exampleof FIG. 3 show two power switch 105 current waveforms 303 and 304. Inboth waveforms 303 and 304, the power switch current ID 302 ramps to afinal value substantially equal to I_(DPKMAX) 305, which is a protectivecurrent limit threshold defining the maximum peak current that can flowin the power switch 105 that is set by for example controller 115 inFIG. 1. As shown, the X % higher energy transfer element input windinginductance L_(I) in the case of waveform 304, requires that the powerswitch 105 be on for X % longer than the power switch 105 is on inwaveform 303 to reach the same protective current limit thresholdI_(DPKMAX) 305. The increase in the power switch 105 on time istherefore directly proportional to the increase in energy transferelement input winding inductance as predicted in the relationship ofequation (1) assuming V_(IN) is constant. If in response to thisincrease in power switch 105 on time, the power switch switching cycleperiod T in equation (4) is also increased by X %, the value ofP_(outmax) will remain substantially constant since L_(I) is also X %higher. Therefore, in the case of energy transfer element input windinginductance L_(I) tolerance, the correct value for factor Ka 309 issubstantially equal to 1.

The waveforms 301 example of FIG. 3 show two power switch 105 currentwaveforms 306 and 307. As shown, current waveforms 306 and 307 increasesubstantially linearly while power switch 105 is on until the currentwaveforms 306 and 307 reach the protective current limit thresholds. Inthe example, waveform 306 has a protective current limit threshold ofI_(DPKMAXnom) 309 while waveform 307 has a protective current limitthreshold of I_(DPKMAXY) 308, which is Y % higher than current limitthreshold I_(DPKMAXnom) 309. Therefore, the power switch 105 on timewill increase by substantially Y % when the protective current limitthreshold increases by Y %.

From the relationship of equation (4) however, the influence of this Y %increase of current limit threshold has a square law effect on themaximum output power capability P_(outmax) of the power converter 100.For example if the I_(DPKMAX) value in equation (4) is increased by 5%,the P_(outmax) value in equation (4) increases by substantially 10%. Thefact that squaring a small percentage change, such as for example ±15%,results in a substantial doubling of the overall percentage change,allows factor Ka 310 in FIG. 3 to be chosen to be substantially equal to2 in order to substantially cancel the effects of changes in the powerswitch protective current limit threshold.

FIG. 4 shows curves for the relationship between power switch switchingcycle period and the product of a signal representative of an inputvoltage to the power converter and an on time period of the power switcht_(on). As discussed above, a value of Ka substantially equal to 1 willsubstantially cancel variations in the energy transfer element inputwinding inductance L_(I) while a value of Ka substantially equal to 2will substantially cancel variations in the protective current limitthreshold of controller 115.

FIG. 5 shows a flowchart of an example method for controlling themaximum output power of a power converter in accordance with theteachings of the present invention. In block 501 the power switch isturned on and in block 502 a timing period is started. It is noted withreference to the above descriptions that although the timing perioddiscussed thus far is the on time period of the power switch, the timingperiod could start and end at any time during the on time of the powerswitch when there are two separate current levels in the power switch.In block 512, the signal representative of an input voltage to the powerconverter is sensed. In block 503, it is determined if the power switchcurrent has changed between two current thresholds since the timingperiod began. In the description so far, the first current threshold hasbeen substantially zero and the second current threshold has been at theprotective current limit threshold. However it is noted that if onlycorrection for the tolerance of the energy transfer element inputwinding inductance is required, two other current threshold values couldbe employed to determine the slope of the power switch current waveformduring the power switch on time. It is noted that if correction fortolerance in the protective current limit threshold is required, atleast one of the current thresholds in block 503 has to be theprotective current limit threshold. In block 504, the timing is stoppedand the product (KV_(IN)×t) of the signal representative of the inputvoltage and the measured time between starting and stopping the timer iscalculated. Blocks 505, 506, 507, 508 and 509 determine the actionrequired based on the calculation of (KV_(IN)×t). In particular, if(KV_(IN)×t) is greater than a nominal value in block 505, the powerswitch switching cycle time is increased in block 507. In one example,the nominal value may be determined at the design stage of the powersupply. In particular, a designer can calculate what the nominal valueswill be to achieve the desired output power. To illustrate, by selectinga resistor R₁₁₁, and therefore K, the designer can choose the nominaloperating period for a given (V_(IN)×t) from FIG. 4. For example thenominal period could be the point (1.1) in FIG. 4. If (KV_(IN)×t) isgreater than nominal (e.g. more than 1 in FIG. 4) the period wouldincrease. Referring back to FIG. 5, if (KV_(IN)×t) is less than nominalin block 506, the power switch switching cycle time is decreased inblock 508. If (KV_(IN)×t) is at the nominal value, no change to theswitching cycle period is made in block 509. Block 510 determineswhether the present switching cycle period is complete before startingthe next power switch switching cycle at block 501.

It is noted that although the above description employs the power switchswitching period as the control parameter for adjustment based on themeasured value of the (KV_(IN)×t) product, more generally the ratio ofthe power switch on time to the power switch off time during any powerswitch switching cycle period, known as the power switch duty cycle, isa broader description of the same control functionality. In general, thepower switch duty cycle can be adjusted by adjusting the power switchswitching cycle period but also by other techniques including adjustingthe power switch protective current limit threshold, directlycontrolling the period of time for which the power switch is on duringeach switching cycle period, on/off control, pulse width modulation orother suitable power converter switching techniques.

FIG. 6 shows a detailed schematic of a portion of a controller 615 thatin one example is equivalent to controller 115 in FIG. 1. Voltage sourceV_(AUX) 603 is in one example equivalent to auxiliary winding voltageV_(AUX) 181 in FIG. 1. In one example the controller 615 is coupled todrive a power switch equivalent to power switch 105 in FIG. 1. In oneexample resistor 601 is equivalent to resistor R₁₁₁ in FIG. 1. In oneexample ground potential terminal 605 is equivalent to ground potentialterminal 124 in FIG. 1. The following description of the circuit 600 ofFIG. 6 references waveforms 700 in FIG. 7.

As shown in FIG. 6, current signal 606 representative of an inputvoltage V_(IN) 101 to the power converter flows during the on time ofthe power switch 105. The coupling of the gates of transistors 632 and633 maintains the voltage at FB terminal 603 substantially equal tocontroller ground potential 605 such that the value of the currentsignal 606 is substantially equal to V_(AUX)/R₆₀₁. Current source 634biases transistor 633. Current 606 also flows in transistor 631, whichforms a current mirror with transistor 637. However, due to the presenceof transistor 636, which is only on for a period that CLK-line 607 ishigh, the voltage on the gate of transistor 631 is sampled and held oncapacitor 663 until the next time CLK-line signal 607 goes high. It willbe noted that CLK-line signal waveform 703 goes high a delay periodafter the start of power switch on time 706. In one example, this delayperiod is substantially equal to 400 nsecs and the duration of CLK-linepulse high is substantially equal to 100 nsecs. The held voltage acrosscapacitor 663 sets the value of current that flows in transistor 637when transistor 639 is on. Transistor 639 is turned on for the durationthat the power switch gate drive signal 608 and 701 is high, which inone example is equivalent to signal 122 in FIG. 1. During the on time oftransistor 639, capacitor 642 is therefore charged with a fixed currentas illustrated by waveform 704 in FIG. 7. Since the value of the chargecurrent 610 is determined by the value of current signal 606 asdescribed above, the slope of rising voltage 707 is responsive to thevalue of the current signal 606, which is representative of the inputvoltage to the power converter. Capacitor 642 is charged at this ratefor the on time period of the power switch such that the voltage acrosscapacitor 642 at the end of the power switch on time period isrepresentative of the product of an input voltage signal and the on timeperiod of the power switch. It is noted that by replacing gate signal608 with a signal that is high only for a portion of the power switch ontime period, a time period other than the complete on time period of thepower switch could be selected. A further gating signal CLK-GF 609 turnson transistor 641 for a short period that may in one example besubstantially 100 nsecs allowing capacitor 643 to sample and hold thevoltage across capacitor 642. A delayed version of CLK-GF signal 609 isapplied to transistor 640 to reset capacitor 642 to be ready for thenext power switch on time during the next switching cycle. Components644, 645, 646, 647, 648 and 649 provide a high impedance buffer circuitto ensure that capacitor 643 is coupled only to high impedance to helpprevent discharge of capacitor 643. This high impedance buffer operatesto provide a voltage across resistor 610 that is substantially equal tothe voltage across capacitor 643. Since a known voltage is thereforeestablished across a known resistor 610, a known current 618 flows intransistor 651 and is mirrored as current 616 flowing in transistor 652.A reference voltage level V_(bg)+V_(be) 612 applied to the base oftransistor 657 applies a voltage V_(bg) across resistor 611, which inone example has substantially equal value to resistor 610, setting up acurrent 619 that is reflected once by transistor 656 and again bytransistors 655 and 654 to provide current 613. It will be noted thatdiode 653 is reverse biased and current 617 therefore substantiallyequal to zero, until current 616 described above exceeds the value ofcurrent 613. For current 616 values in excess of current 613, current617 contributes to current 677 flowing in resistor 661, which in turnadjusts the oscillator voltage level and therefore oscillator frequencythat is the reciprocal of the power switch switching cycle period. Inone example oscillator 604 includes a capacitor that is charged anddischarged between the two node voltages of resistor 661.

It is noted that the action of current 613 and diode 653 ensure thatcurrent 616 has no influence on the power switch switching cycle periodbelow a threshold value. This limits the tolerance of energy transferelement input winding inductance and controller protective current limitthreshold that can be compensated for.

From the circuit description above, it is clear that since the degree ofinfluence current 677 has on the oscillator switching frequency is afunction of the value of resistor 661 that the value of Ka in FIG. 4 issubstantially fixed. It is therefore necessary to choose whether thecontroller is to correct substantially completely for energy transferelement inductance and therefore only partially for tolerance in theprotective current limit threshold. Alternatively the choice of resistor661 could be made to correct substantially completely for the protectivecurrent limit threshold and therefore over compensate for energytransfer element inductance tolerance. In a practical circuit, a choicemay be made to provide a compromise somewhere between the values of Ka=1and Ka=2 illustrated in FIG. 4. One example practical choice for thevalue of Ka would be substantially equal to 1.3.

With reference to the circuit of FIG. 1, the circuit of FIG. 6 includesa sensor circuit 699, which in one example is equivalent to sensorcircuit 141, a timing and multiplier circuit 698, which in one exampleis equivalent to timing and multiplier circuit 142 and an oscillator604, which in one example is equivalent to oscillator circuit 143. Thecircuit of FIG. 6 also shows a signal 691 that in one example isequivalent to signal 146 in FIG. 1, which is coupled to the drive signalgenerator circuit 144, and a signal 608, which in one example isequivalent to the output signal 145 from current sense circuit 140 inFIG. 1.

In the specific example of FIG. 6, since the controller gate drivesignal 608 is generated when a protective current limit threshold hasbeen reached, the gate drive to the power switch 105 is equivalent tothe output signal 145 in FIG. 1. It is noted, however, that if currentthreshold values other than the protective current limit threshold wereused to control timing and multiplier circuit 142 with signal 145, thatsignal 145 would not correspond to the power switch gate drive signal608.

The above description therefore illustrates the detailed realization ofa circuit implementation that may form a portion of a controller coupledto a power switch and coupled to receive an input voltage signalrepresentative of an input voltage to the power supply. A time periodbeing the time for which the power switch is in an on state which in oneexample is a time taken for a current flowing in the power switch tochange between two current values, the controller to adjust a switchingcycle period of the power switch to be proportional to the product ofthe input voltage signal and the time period.

In the above description of control circuit 615, an oscillator 604period is responsive to a voltage across a resistor 661. It is notedhowever that in another example the oscillator period couldalternatively be responsive to a value of a digital counter circuitwhile still benefiting from the teachings of the present invention. Inone example, a digital counter circuit could be incremented at afrequency higher than the power switch switching frequency, responsiveto the value of the input signal representative of the input voltage tothe power converter. The value of the digital counter count could thenbe compared to a threshold number to set the oscillator 604 frequency inthe following switching cycle. It is noted that other techniques couldbe used as alternatives to the above descriptions for controlling theoscillator period while still benefiting from the broader teachings ofthe present invention.

It is noted that the circuit realization described in FIG. 6 provides acycle by cycle adjustment of the oscillator period. It is noted thatalternatives to the above descriptions where the oscillator period isadjusted over multiple cycles could be used while still benefiting fromthe broader teachings of the present invention.

The preceding description describes techniques whereby the power switchswitching cycle period is adjusted in response to a product of a signalrepresentative of an input voltage and a time period taken for a currentflowing in the power switch to change between two threshold values. Itis noted however that an equivalent method to achieve the similarfunctionality would be to measure the time taken for a current flowingin the power switch to change between two threshold values and comparethis time with an expected or control time period that is responsive toan input voltage signal representative of an input voltage to the powersupply. Then to adjust a power switch duty cycle in response to thedifference between the measured and control time periods. In one examplethe method used to adjust power switch duty cycle is to adjust the powerswitch switching cycle period. The flowchart of FIG. 8 illustrates sucha method. The power switch is turned on in block 801 and the signalrepresentative of the input voltage to the power supply is sensed inblock 802. A timer is started in 812 and a determination of whether thecurrent in the power switch has changed between two threshold values ismade in block 803. In block 804, the timing is stopped and t_(measured)is calculated. In block 811, the expected or control time period,t_(control), is calculated. In blocks 805, 806, 807, 808 and 809 thepower switch switching cycle period is adjusted in response to thecomparison between t_(control) and t_(measured). In particular, ift_(measured)>t_(control) in block 805, the power switch switching cycleperiod is increased in block 807. If t_(measured)<t_(control) in block806, the power switch switching cycle period is decreased in block 808.If t_(measured) is substantially equal to t_(control) the power switchswitching cycle period is left unchanged in block 809.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific voltages,currents, frequencies, power range values, times, etc., are provided forexplanation purposes and that other values may also be employed in otherembodiments and examples in accordance with the teachings of the presentinvention.

These modifications can be made to examples of the invention in light ofthe above detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation. The present specification and figures are accordingly tobe regarded as illustrative rather than restrictive.

What is claimed is:
 1. A control circuit for use in a power converter,the control circuit comprising: an input voltage sensor coupled togenerate a first signal representative of an input voltage (Vin) of thepower converter; a current sensor coupled to generate a second signalrepresentative of a switch current through a power switch of the powerconverter; and a drive signal generator configured to generate a drivesignal to control switching of the power switch in response to the firstand second signals, wherein the drive signal generator adjusts a dutycycle of the drive signal based on a product K×Vin×t to control amaximum output power of the power converter, wherein K is a fixed numberand t is a time it takes the second signal to change between two valuesof the switch current when the power switch is in an on state.
 2. Thecontrol circuit of claim 1, wherein the fixed number K is substantiallyequal to 1 to compensate for variances in an inductance value of anenergy transfer element of the power converter.
 3. The control circuitof claim 1, wherein the fixed number K is substantially equal to 2 tocompensate for variances in a protective current limit threshold of thepower switch.
 4. The control circuit of claim 1, wherein the fixednumber K is a number between the values of 1 and 2 to compensate forvariances in both an inductance value of an energy transfer element ofthe power converter and a protective current limit threshold of thepower switch.
 5. The control circuit of claim 4, wherein the fixednumber K is substantially equal to 1.3.
 6. The control circuit of claim1, wherein one of the two values of switch current is substantiallyzero.
 7. The control circuit of claim 1, wherein one of the two valuesof switch current is a protective current limit threshold of the powerswitch.
 8. The control circuit of claim 1, wherein the drive signalgenerator is configured to adjust the duty cycle of the drive signal byadjusting a power switch protective current limit threshold of the powerswitch.
 9. The control circuit of claim 1, wherein the drive signalgenerator is configured to adjust the duty cycle of the drive signal bycontrolling a period of time that the power switch is in the on stateduring each switching cycle period.
 10. The control circuit of claim 1,wherein the drive signal generator generates the drive signal to controlswitching of the switch to regulate an output of power converter inresponse to a feedback signal.
 11. A power converter, comprising: anenergy transfer element coupled between an input and an output of thepower converter; a power switch coupled to the energy transfer element;and a control circuit coupled to control switching of the switch toregulate the output of the power converter, wherein the control circuitincludes: an input voltage sensor coupled to generate a first signalrepresentative of an input voltage (Vin) of the power converter; acurrent sensor coupled to generate a second signal representative of aswitch current through a power switch of the power converter; and adrive signal generator configured to generate a drive signal to controlswitching of the power switch in response to the first and secondsignals, wherein the drive signal generator adjusts a duty cycle of thedrive signal based on a product K×Vin×t to control a maximum outputpower of the power converter, wherein K is a fixed number and t is atime it takes the second signal to change between two values of theswitch current when the power switch is in an on state.
 12. The powerconverter of claim 11, wherein the fixed number K is substantially equalto 1 to compensate for variances in an inductance value of an energytransfer element of the power converter.
 13. The power converter ofclaim 11, wherein the fixed number K is substantially equal to 2 tocompensate for variances in a protective current limit threshold of thepower switch.
 14. The power converter of claim 11, wherein the fixednumber K is a number between the values of 1 and 2 to compensate forvariances in both an inductance value of an energy transfer element ofthe power converter and a protective current limit threshold of thepower switch.
 15. The power converter of claim 14, wherein the fixednumber K is substantially equal to 1.3.
 16. The power converter of claim11, wherein one of the two values of switch current is substantiallyzero.
 17. The power converter of claim 11, wherein one of the two valuesof switch current is a protective current limit threshold of the powerswitch.